The present invention relates to a method and apparatus for controlling an ink film thickness in an ink supply unit (inker) which supplies an ink in an ink fountain to a printing plate through an ink roller group.
FIG. 9 shows the main part of an ink supply unit in each printing unit of a web offset printing press. An ink supply unit 11 shown in FIG. 9 has an ink fountain 1, an ink 2 stored in the ink fountain 1, an ink fountain roller 3, a plurality of ink fountain keys 4, an ink ductor roller 5, an ink roller group 6, a printing plate 7, a plate cylinder 8, a plurality of nozzles 9, and an ink roller cleaning unit 10. The ink fountain keys 4 are aligned in the axial direction of the ink fountain roller 3. The nozzles 9 are arranged in the axial direction of the ink roller group 6 to discharge a solvent.
Through opening degree adjustment of the ink fountain keys 4, the ink supply unit 11 having the above arrangement supplies the ink 2 in the ink fountain 1 to the ink fountain roller 3, and supplies the ink, which has been supplied to the ink fountain roller 3, to the printing plate 7 through the ink roller group 6 upon the feed operation of the ink ductor roller 5.
When an old printing plate is exchanged for a new printing plate 7, the opening degree of the ink fountain keys 4, the rotation ratio of the ink fountain roller 3, and the like are preset in accordance with the image of the new printing plate 7. More specifically, after the opening degree of the ink fountain keys 4, the rotation ratio of the ink fountain roller 3, and the like are set in accordance with the image of the new printing plate 7, the ink 2 in the ink fountain 1 is supplied to the new printing plate 7 through the ink roller group 6. In this case, test printing is performed before final printing to adjust the ink supply amount, thus obtaining a satisfactory color tone. A desired ink film thickness distribution (gradient of the ink film thickness) is accordingly formed in the ink roller group 6.
This pre-inking operation is already described in U.S. Ser. No. 08/884,348, now U.S. Pat. No. 5,884,562 and U.S. Ser. No. 08/884,349 filed by the present applicant.
In the conventional ink supply unit 11, when the old printing plate is exchanged for the new printing plate 7, an ink film thickness distribution corresponding to the old printing plate remains in the ink roller group 6. In this case, color change is performed or not, depending on the new printing plate 7.
When color change is not performed, while the ink is left in the ink roller group 6, the ink film thickness distribution corresponding to the old printing plate is gradually changed to an ink film thickness distribution corresponding to the new printing plate 7. Accordingly, adjustment of the ink supply amount and test printing are required excessively until a satisfactory color tone is obtained, causing an increase in pre-printing preparation time, an increase in labor load, waste of printing material, a decrease in production efficiency, an increase in cost, and the like.
In the color change, an ink cleaning operation is performed, and the ink in the ink fountain 1 is exchanged. Thus, the ink roller group 6 no longer holds an ink, and an ink film thickness distribution corresponding to the new printing plate 7 must be formed in the ink roller group 6 from the beginning. Therefore, it takes time until the ink film thickness distribution reaches an equilibrium state, causing an increase in pre-printing preparation time, an increase in labor load, waste of printing material, a decrease in production efficiency, an increase in cost, and the like.
In conventional ink film thickness control, a time-series ink film thickness distribution forming operation is performed for each printing unit independently. This delays start of final printing, interferes with the productivity, and increases the production cost. The ink film thickness distribution forming operation differs from one printing unit to another to require an complicated operation, and accordingly imposes an excessive work load to the operator. Since similar operations are performed repeatedly, operation errors are difficult to prevent. A work loss always occurs, and an unnecessary ink film thickness correcting operation is performed.